232 



THE CAT. 



[chap. VIII. 



different secretions as are those, for example, of the sweat glands, 

 salivary glands, synovial membrane, and liver. _ The undivided tube 

 of a gland by which its secretion is poured out is its duct. The se- 

 cretions, and therefore their glands may, as we have seen, simply 

 serve to aid the process of assimilation. They may also aid the 

 function of generation, or, finally, they may merely serve for ex- 

 cretion, i. e., to get rid of waste products or excreta. Certain large 

 and small glands have already been described in the sixth chapter, 

 namely, the liver, the pancreas and the various salivary glands. 

 The anal glands were also therein noticed. It remains to describe 

 those very important glands, the kidneys. 



§ 9. As the foods and the tissues of the body may both be 

 divided into nitrogenous and non-nitrogen ons substances, so also 

 the excreta of the body may be similarly divided. The non- 



Fig. 109.— The Cat's Kidney, entire and in section. 



A. Tlio outer surfaf e of tlu! kidney, showing the 



network of bluud-vessels. 

 (I. Renal artery. 

 V. Ureter. 

 V. Renal vein. 



B. Vertieal seetion through the kidney. 



c. Cortical suhstaiice. 



■i. Expanded end (jf the calix .survoundiug 



the mainniilla. 

 0. Dark spots in cortical substance. 

 p. Papilla, or niannnilla. 

 })/. Pelvis. 

 /. Tubules of the kidney. 



nitrofjciwiiR products of waste arc eliminated by the lungs, and to a 

 very much less degree by the skin in the fonn of water and carbonic 

 acid. But a very large portion of the waste products are nitrogenous. 

 These are eliminated in a trifling degree also by the skin, but the 

 special organs for their elimination are the rcmd organs or kidiir/js. 

 A process of oxidation in the innermost substance of the body 

 converts the nitrogenous waste matter into urea, uric acid, ammonia, 

 and certain other acids and salts which are crystalloidal derivations 

 from colloidal tissues. The kidneys extract all these, with much 

 water, frum the blood, and so form uiinc. 



§ 10. The KIDNEYS differ from the lungs in that they arc organs 

 of excretion only. The lungs excrete, but as we have just seen, they 

 also take in. The secretion of the kidney, the urine, passes down 

 from those organs by two tubes into a receptacle — the bladder — where 

 it accumulates, and whence it is expelled at intervals. 



The kidneys are two ^organs placed one on either side of the 



